The invention relates to the structure of a superheater of a steam boiler. Superheaters of steam boilers are typically placed in a flue gas flow and in circulating fluidized bed boilers (CFB-boiler) superheaters or a part of the superheaters can be placed below the cyclone, in a so-called loopseal (sand seal). The Increase of the superheating temperature and the heat-to-power ratio of the plant are for their part limited by superheater corrosion. The corrosion mechanism varies depending on combustion, structure and most of all the chemical composition of ash and combustion gases.
In boilers using waste and biomass a high content of chlorine (Cl) combined with a high alkali content—which is primarily formed of sodium (Na) and potassium (K)—may lead to a heavy fouling and corrosion of the heat exchange surfaces. Waste and biomass type fuels are especially problematic, because typically their sulphur content (S) is low in relation to their chlorine content, in which case the alkali form alkali chlorides and not alkali sulphates. The compounds being created, in turn, typically have a relatively low melting temperature. The smelt material being created adheres onto the surface of the superheater and creates corrosion. Several other compounds created in the combustion process have corresponding properties as well.
Corrosion is aimed to be controlled by selecting materials that endure corrosion better either over the entire thickness of the material or for the part of the surface layer of the pipe. In addition, corrosion is aimed to be decreased by designing the surface temperature of the superheater below the melting temperature. A low temperature of the superheated steam is not advantageous from the point of view of the operational economy of the plant (lower electricity production).
The surface temperature of the material of a typical superheater is, by means of the present technique, a few tens of degrees higher than the temperature of the contents, depending on the conditions. In practice, the surface temperature and corrosion rate of the material can be substantially affected only by changing the temperature of the contents, i.e. by limiting the superheating temperature.
A superheater material that must simultaneously endure corrosion, high pressure and high temperature, is typically expensive.